Osteoporosis is a bone disease affecting nearly 1 in 3 women worldwide and over 55% of all Americans resulting in health care costs exceeding 19 billion dollars annually in the United States alone. Since the discovery of the TGF? inducible early gene-1 (TIEG) by our laboratory, we have demonstrated that loss of TIEG expression in mice results in multiple defects in osteoblasts and osteoclasts and confers a gender specific osteopenic phenotype affecting only female animals. Recently, TIEG has been identified as one of only a few genes whose altered expression levels or allelelic variations associate with decreased bone mass and osteoporosis in humans. During the past funding period, we have shown that TIEG expression is essential for estrogen signaling in bone and have identified a novel role for TIEG in mediating canonical Wnt signaling in the skeleton. These observations are of significant interest since estrogen remains one of the most important regulators of bone homeostasis in both men and women and since Wnt signaling is essential for normal bone development and maintenance. For these reasons, it is crucial to understand the regulation of these pathways in the skeleton. In this proposal, we present evidence that TIEG enhances the Wnt signaling pathway via dual mechanisms whereby it directly suppresses the expression of sclerostin and serves as a transcriptional co- activator for ?-catenin. Furthermore, our data suggest that TIEG serves as a direct link between the estrogen and canonical Wnt pathways in bone. Based on these observations, our central hypothesis is that TIEG is a crucial component of canonical Wnt signaling and serves a central role in regulating cross-talk between the estrogen and Wnt pathways in the skeleton. In order to test this hypothesis, the following Specific Aims are proposed: 1): Characterize the regulation of sclerostin gene expression by TIEG and determine its contribution to the observed osteopenic phenotype of TIEG knockout mice; 2): Characterize the ability of TIEG to enhance canonical Wnt signaling through co-activation of ?-catenin; and 3) Determine if the gender specific (female only) osteopenic phenotype of TIEG KO mice results from defective cross-talk between the estrogen and canonical Wnt signaling pathways in bone. To address these Specific Aims, we will employ in vitro approaches to identify the molecular mechanisms by which TIEG suppresses sclerostin expression in osteocytes and enhances ?-catenin function in osteoblasts. Additionally, we will utilize multiple in vivo models and approaches to dissect the role of TIEG in regulating canonical Wnt pathway activity and cross-talk between estrogen and Wnt signaling in the skeleton. Considering the fundamental roles of estrogen and Wnt signaling in bone, as well as the present day targeting of these pathways for the treatment of osteoporosis, it is essential to understand the role of TIEG in regulating these two pathways. Completion of the proposed studies is therefore expected to provide significant knowledge with regard to developing novel strategies to treat this debilitating disease.